1. Field of the Invention
The present invention relates to an image forming apparatus, a method for forming an image, a computer-readable storage medium, and a computer system.
2. Description of the Related Art
(1) Image forming apparatuses that use toner, for example, as the developer and that form images by developing latent images formed on a photoconductor, which serves as an image bearing member, are known as a type of image forming apparatus that has a turning member on which a plurality of developing devices are mounted and that causes the developer, which is contained in a container of each developing device, to move along with the turning movement of the turning member. In such image forming apparatuses, each of the developing devices contains different colors of toner, such as yellow toner, magenta toner, cyan toner, and black toner. Each of the developing devices are made to oppose the photoconductor one by one, and a latent image formed on the photoconductor is developed by supplying the toner in the developing device opposing the photoconductor, using a developer-supplying section provided in that developing device. The developed toner image is transferred onto an intermediate medium and then onto paper, for example, which serves as a medium. During these processes, the toner is charged by friction, for example, in order to make the toner adhere to the photoconductor.
When printing a color image, each of the developing devices is placed opposed to the photoconductor successively through the turning movement of the turning member in order to form single-color toner images in each color. The single-color toner images in four colors are transferred onto the intermediate medium in a superimposed manner, and then the superimposed color toner image is transferred onto paper. (Refer to, for example, Japanese Patent Application Laid-open Publication No. 2000-347499.)
However, if, for example, the image forming apparatus described above is used to continuously print single-color images, such as texts, on a multitude of sheets of paper using only black toner, the developing device containing black toner will be kept in the position opposing the photoconductor during the continuous printing. In other words, during the continuous printing, the turning member does not turn. As a result, in image forming apparatuses that cause the developer contained in the container of each developing device to move along with the turning movement of the turning member, the toner in the developing devices will not be stirred during the continuous printing using a single type of toner.
The toner on the side of the developer supplying section is charged and supplied to the photoconductor. However, particularly when printing monochrome images such as texts, a major portion of the charged toner is returned back into the developing device without adhering to the photoconductor. If the developing process continues without the toner in the developing device being stirred, only the toner on the side of the developer supplying section will deteriorate. That is, toners having significantly different characteristics will be contained in the developing device. When toners of different characteristics are mixed, problems such as fogging in images, scattering of toner, and toner spill tend to occur.
(2) A color image forming apparatus comprising a rotary-type developing unit that has a plurality of developing devices for developing a latent image formed on a photoconductor (which serves as an example of an image bearing member) using toner (as an example of developer) and in which these developing devices are arranged radially about the axis of rotation of the developing unit, is also known as another type of image forming apparatus. When image signals are sent from an external device, such as a host computer, to this kind of color image forming apparatus, the apparatus positions one of the developing devices in the developing position opposing the photoconductor by making the developing unit rotate about its rotation axis. In this position, the image forming apparatus develops a latent image formed on the photoconductor with the developing device to form a toner image, and transfers the toner image onto an intermediate transferring element, which serves as an example of a transferring medium. The above-mentioned developing and transferring processes are repeated while successively changing the positions of the developing devices, and a plurality of toner images are transferred onto the intermediate transferring element in a superimposing manner, forming a color image. Finally, the color image is transferred onto a transferring material such as paper. (Refer to, for example, Japanese Patent Application Laid-open Publication No. 6-348100 and Japanese Patent Application Laid-open Publication No. 5-107864.)
When forming a color image using such a color image forming apparatus by superimposing toner of several colors onto the intermediate transferring element, a phenomenon in which toner of one color inadvertently overlies toner of another color that has already been transferred onto the intermediate transferring element sometimes occurs. (This phenomenon is called “fogging” herein.) “Fogging” causes deterioration in the quality of the color image that is finally transferred onto the transferring material. For this reason, there has been a demand for a technique to reduce occurrence of “fogging”.
Furthermore, when transferring toner onto the intermediate transferring element, a phenomenon called “hollow defects” sometimes occurs. Hollow defects cause deterioration in the quality of the color image that is finally transferred onto the transferring material. For this reason, there has also been a demand for a technique to reduce occurrence of hollow defects.